An Oxygen-peri-Bridged Quinolinium Cation and Its Monoradical Counterpart

Article abstract of DOI:10.1002/ejoc.201601572

Generation of a σ,σ,π-triradical was attempted by collision-activated dissociation (CAD) of two iodine atoms and an NO^· radical from 2,4-diiodo-5-nitroquinolinium cation in an ion trap mass spectrometer. However, the reactivity of the product was consistent with a monoradical rather than a triradical. The mechanism of its formation was explored. In the first CAD step, an iodine atom is lost to generate the 2-iodo-5-nitro-4-dehydroquinolinium cation. Quantum chemical calculations suggest that in the second CAD step, the radical site at C-4 adds to the adjacent NO₂ group followed by loss of NO^· to generate the 2-iodo-4-oxyl-5-dehydroquinolinium cation. Calculations also suggest that in the third CAD step, this cation cyclizes to form an oxygen-peri-bridged 2-iodoquinolinium cation that then loses the remaining iodine atom to generate a distonic 4,5-oxygen-peri-bridged 2-dehydroquinolinium cation. For the analogous 4-oxyl-5-dehydroquinolinium cation, the cyclization is calculated to be exoergic by 8.7 kcal mol^–1, with a free energy barrier of 14.0 kcal mol^–1. The cyclized product is the first experimentally observed oxygen-peri-bridged naphthalene derivative. It displays low reactivity. However, it's monoradical counterpart is highly reactive and undergoes typical radical reactions.

Full text of DOI:10.1002/ejoc.201601572

A Journal of
Accepted Article
Title: An Oxygen-Peribridged Quinolinium Cation and Its Monoradical
Counterpart
Authors: Raghu Kotha, John Nash, and Hilkka Kenttamaa
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To be cited as: Eur. J. Org. Chem. 10.1002/ejoc.201601572
Link to VoR: http://dx.doi.org/10.1002/ejoc.201601572
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10.1002/ejoc.201601572
European Journal of Organic Chemistry
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An Oxygen-Peribridged Quinolinium Cation and Its Monoradical
Counterpart
Raghavendhar R. Kotha, [a] John J. Nash, [a] and Hilkka I. Kenttämaa*[a]
Abstract: Generation of a σ,σ,π-triradical was attempted by collision-
activated dissociation (CAD) of two iodine atoms and an NO● radical
from 2,4-diiodo-5-nitroquinolinium cation in an ion trap mass
spectrometer. However, the reactivity of the product was consistent
with a monoradical rather than a triradical. The mechanism of its
formation was explored. In the first CAD step, an iodine atom is lost
to generate the 2-iodo-5-nitro-4-dehydroquinolinium cation. Quantum
chemical calculations suggest that in the second CAD step, the radical
site at C-4 adds to the adjacent NO2 group followed by loss of NO● to
generate the 2-iodo-4-oxyl-5-dehydroquinolinium cation. Calculations
also suggest that in the third CAD step, this cation cyclizes to form an
oxygen-peribridged 2-iodoquinolinium cation that then loses the
remaining iodine atom to generate a distonic 4,5-oxygen-peribridged
2-dehydroquinolinium cation. For the analogous 4-oxyl-5-
dehydroquinolinium cation, the cyclization is calculated to be exoergic
by 8.7 kcal mol-1 with a free energy barrier of 14.0 kcal mol-1. The
cyclized product is the first experimentally observed oxygen-
peribridged naphthalene derivative. It displays low reactivity. However,
it’s monoradical counterpart is highly reactive and undergoes typical
radical reactions.
neutral biradicals.[11–13] The distonic ion approach has been used
to study a wide range of (charged) organic radicals, including
pyridine-based mono-, bi-, tri- and tetraradicals.[10,14–17]
Aromatic radicals with both σ- and π-type radical sites are
interesting because the odd spin of π-type radical sites is
delocalized into the aromatic system.[18] This spin delocalization
alters the electronic properties of the σ-system, which should
result in changes in reactivity.[19] In order to explore this possibility,
an attempt was made to generate a σ,σ,π-triradical cation in a
linear quadrupole ion trap mass spectrometer by using 2,4-diiodo-
5-nitroquinoline (Scheme 1) and to study its gas-phase reactivity.
Scheme 1. Proposed generation of σ,σ,π-triradical 1a from protonated 2,4-
diiodo-5-nitroquinoline 1 via three sequential collision-activated dissociations in
a linear quadrupole ion trap (LQIT) mass spectrometer.
Introduction
Upon collision-activated dissociation (CAD), iodine atoms
are readily cleaved from protonated aromatic radical precursors
to generate carbon-centered σ-radical sites.[14–17,20,21] On the other
hand, protonated aromatic nitro compounds can lose either a nitro
Organic bi- and polyradicals play an important role in various
fields, including organic synthesis, organic polymers and drug
design.[1–5] For example, enediyne prodrugs, such as
calicheamicins and dynemicins, that have tremendous anticancer
activity generate biologically active para-benzyne analogs
(aromatic carbon-centered σ-type 1,4-biradicals) that rapidly
cleave double-stranded DNA in an irreversible manner.[3]
However, the clinical use of these drugs is limited due to their high
cytotoxicity.[6] Hence, it is important to better understand the
behavior of organic bi- and polyradicals. Arynes (aromatic carbon-
centered σ,σ-biradicals) have been extensively studied both
theoretically and experimentally.[7–9] However, limited data are
available on the reactivities of many other organic bi- and
polyradicals due to challenges in generating such highly reactive
species cleanly in solution.[1,10] One way to address this issue is
to study such radicals in the gas phase (i.e., in a mass
spectrometer) by using the distonic ion approach.[10] In this
approach, a chemically inert charged site is attached to the bi- or
polyradical of interest in order to allow mass spectrometric
manipulation.[10] Previous studies using the distonic ion approach
have shown that the gas-phase reactivity of 1,2-didehydroarenes,
for example, is similar to the solution reactivity of analogous
●
radical (NO2 ) or a nitrosyl radical (NO●) upon CAD.[22–24] The latter
loss results in a π-type oxygen-centered radical site (i.e., a
phenoxyl-type radical).[22–24] Interestingly, CAD of 2,4-diiodo-5-
nitroquinolinium cation differed from what was expected and
ultimately led to the formation of an unprecedented radical, a
distonic oxygen-peribridged quinolinium radical cation, as
described below. The gas-phase reactivity of this species as well
as its even-electron analog were examined and compared to
those of related mono- and biradicals.
Results and Discussion
The radical precursors shown in Figure 1 were used to attempt
the generation of the cations shown in Figure 2. Upon CAD,
protonated 3, 5, and 6 fragmented in a manner expected based
●
on earlier studies (i.e., via facile loss of an iodine atom or NO2
radical along with a minor loss of NO● radical), or loss of a NO2
●
radical followed by loss of an iodine atom) to generate one
[a]
R. R. Kotha, Dr. J. J. Nash, Prof. H. I. Kenttämaa
Department of Chemistry
Purdue University
560 Oval Drive, West Lafayette, IN 47907-2084
E-mail: hilkka@purdue.edu;
https://www.chem.purdue.edu/hilkka
Supporting information for this article is given via a link at the end of
the document.
Figure 1. Radical precursors (1-7) used in the current study.
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10.1002/ejoc.201601572
European Journal of Organic Chemistry
FULL PAPER
of m/z 115; Figure 3). This was not surprising as CO loss is
common for CAD of ionized phenoxyl radicals.[25]
The mechanism of the unexpected, exclusive NO● radical
elimination during the second CAD step for protonated 1 and 2
was examined computationally for protonated 2 (4-iodo-5-
nitroquinolinium cation; Figure 1). DFT calculations indicate that
the loss of NO● is initiated by addition of the C-4 radical site to the
adjacent NO2 group, which is followed by NO● loss, leading to the
4-oxylbiradical (2a’; Figure 4) rather than the expected 5-
oxylbiradical (2a; Figure 2). Assuming analogous behaviour for
protonated 1, the triradical generated upon three CAD steps
should be the 4-oxyl-2,5-didehydroquinolinium cation (1a’; Table
1) instead of the expected 5-oxyl-2,4-didehydroquinolinium cation
(1a; Scheme 1 and Figure 2).
Figure 2. Structures of the radical cations of interest.
(monoradicals 3a and 5a) or two carbon-centered σ-radical sites
(biradical 6a; Figure 2). Protonated 4 lost a ●CH3 radical to
generate an oxyl-radical site (4a), as expected. Protonated 7 lost
first an NO● radical and then an iodine atom (to yield 7a).
●
Protonated 2 was expected to fragment via elimination of NO2
and NO● radicals in the first CAD step but instead it predominantly
lost an iodine atom. This was quite unexpected but can be
rationalized by the (calculated; M06-2X/6-311G(d,p)//M06-2X/6-
311G(d,p)) homolytic bond dissociation energies (BDE) for
protonated 2: the C4-I bond has a BDE (54.0 kcal mol-1) that is ca.
1 kcal mol-1 smaller than that of the C5-NO2 bond. The second
CAD step also gave unexpected results. Instead of the
●
expected[22–24] loss of both NO2 (major) and NO● (minor),
exclusive loss of NO● was observed. The reasons for this
surprising behavior are discussed below.
Similarly to 2, the first CAD step for protonated 1 resulted in
the predominant loss of an iodine atom from C-4 to yield ions of
m/z 300 (Figure 3; the absence of an additional product ion
formed via HI loss demonstrates[16] that the iodine atom loss did
not occur at C-2). Subsequent CAD of the ions of m/z 300
occurred exclusively via
Figure 4. Proposed mechanism for NO● loss from protonated 5-nitro-4-
dehydroquinoline (a) upon CAD to form the 4-oxyl-5-dehydroquinolinium cation
(2a’). Note that once ion a has enough energy to overcome the cyclization
barrier, it also has enough energy to fragment by loss of NO●. Free energies
(G298) calculated at the M06-2X/6-311+G(d,p)//M06-2X/6-311+G(d,p) level of
theory.
The gas-phase reactivities of the mono- and biradicals known
to have structures 3a - 7a and the bi- and triradicals expected to
have structures 1a’ and 2a’ were examined toward dimethyl
disulfide (DMDS; Table 1). Monoradical 5a reacts at 39%
efficiency (i.e., 39% of the collisions between 5a and DMDS result
●
in a reaction) via exclusive abstraction of a SCH3 radical, which
indicates[10] the presence of only one carbon-centered σ-radical
site in 5a, as expected. A similar finding was made for 3a (major
●SCH3 radical abstraction) although this monoradical also
undergoes other reactions that do not indicate the number of
radical sites (Table 1).[10,14,15] Monoradical 4a reacts at 94%
efficiency via almost exclusive H atom abstraction, a reaction
indicative of one oxygen-centered radical site. The high reaction
efficiencies of 3a and 4a (Table 1) are likely a result of their high
(calculated) vertical electron affinities. The higher the electron
affinity of a radical, the more polar the transition states of its
reactions, which leads to enhanced reactivity.[26] Monoradical 5a
Figure 3. CAD of protonated 1 (ions of m/z 427) yields ions of m/z 300 via I atom
loss (top). CAD of ions of m/z 300 yields ions of m/z 270 via NO● loss (middle).
CAD of ions of m/z 270 yields ions of m/z 242 via CO loss (major product), ions
of m/z 143 via I atom loss (the ions of interest here) and ions of m/z 115 via both
I atom loss and CO loss (bottom).
the loss of a NO● radical to yield ions of m/z 270 (also similar to
2). CAD of the ions of m/z 270 resulted in the elimination of the
remaining iodine atom to yield ions of m/z 143, as expected. CAD
of the ions of m/z 270 also resulted in the loss of CO (to yield ions
of m/z 242), and loss of both CO and an iodine atom (to yield ions
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10.1002/ejoc.201601572
European Journal of Organic Chemistry
FULL PAPER
Table 1. Calculated Vertical Electron Affinities (EA) and Singlet-Triplet Splittingsa of Radicals 2a’ and 3a – 7a and the Efficienciesb,c and Products (with
Their Branching Ratios) for Reactions of Radicals 1a’, 2a’ and 3a – 7a With Dimethyldisulfide
7a
2a‘ (?)
1a‘ (?)
3a
4a
5a
6a
EA: 6.39 eV
EA: 8.13 eV
EA: 8.07 eV
EA: 6.30 eV
EA: 8.00 eV
EA: 5.06 eV
-
ES-T: -0.3 kcal mol-1
ES-T: -2.1 kcal mol-1
ES-T: -0.9 kcal mol-1
SCH3 abs 96%
SCH3 abs 76%
HSCH3 abs 24%
SCH3 abs
H abs
e - abs
86%
7%
6%
H abs
e - abs
SCH3 abs trace
100%
trace
SCH3 abs 100%
Efficiency = 39%
SCH3 abs 97%
(2o) SCH3 abs
SSCH3 abs 3%
HSCH3 abs 55%
2 SCH3 abs 19%
SCH3 abs 15%
CH2S abs 10%
e - abs
4%
Efficiency = 100%
Efficiency = 1%
Efficiency = 99%
Efficiency = 94%
Efficiency = 49%d
Efficiency = 44%
aCalculated at the CASPT2/cc-pVTZ//UB3LYP/cc-pVTZ level of theory. bReaction efficiency (% of collisions leading to reaction) = kexp/kcollision x100; precision
+ 10%; accuracy + 50%. cabs = abstraction. dLess reactive isomer: 31% ion population, efficiency = 7%.
has a lower reaction efficiency compared to the other mono-
radicals because of its much lower (calculated) vertical electron
affinity (Table 1).
pathways (branching ratios: 29%, 16% and 55%, respectively).
The low reactivity observed for these three ionic species suggests
that most of these cations do not have the expected structures
because carbon-centered σ–radicals are known to react rapidly
with DMDS (e.g., 3a and 5a; Table 1). Quantum chemical (DFT)
calculations suggest that these cations have undergone ring
closure (for a, see Figure 4) prior to reactions with DMDS. The
ring closure has a calculated free energy barrier of 14.1 kcal mol-
As expected based on the results for the monoradicals,
biradical 7a (calculated S-T splitting: -2.1 kcal mol-1; Table 1)
reacts predominantly via H atom abstraction by the oxygen radical
●
site and SCH3 abstraction by the carbon radical site to yield a
1
HSCH3 abstraction product via elimination of CH2=S. One and two
●SCH3 abstractions and CH2=S abstraction were also observed.
Biradical 6a (calculated S-T splitting: -0.3 kcal mol-1; Table 1)
reacts via predominant abstraction of a ●SCH3 group followed by
another ●SCH3 group abstraction (Table 1), in agreement with
literature.[10,14,15] Both biradicals show lower reaction efficiencies
than monoradical 3a, which may be due to the (weak) coupling
between the radical sites. As expected based on their calculated
S-T splittings, 6a with a smaller splitting reacts slightly faster.
(Figure 4). This ring closure likely occurs within the collision
complex of the cations with DMDS before any other reactions
occur. This type of gas-phase collision complex typically contains
15-25 kcal mol-1 of internal energy released upon solvation of the
cation with the neutral reagent molecule,[27] which makes the
cyclization feasible.
Unfortunately, the experimental results did not provide
information that could be used to rationalize the low reactivity
observed for 2a’. Hence, additional DFT calculations were carried
out. The DFT calculations suggest that the 4-oxyl- and C-5 radical
sites in 2a’ can recombine to form a four-membered ring (2b;
Scheme 2), thus generating a 4,5-oxygen-peribridged quinolinium
cation that has 8.7 kcal mol-1 lower free energy than the biradical
structure (2a; Scheme 2). This cyclization has a calculated free
energy barrier of 14.0 kcal mol-1. Thus, as discussed above for
the monoradical precursors of 1a’ and 2a’, this rearrangement
also likely occurs within the collision complex of 2a’ with DMDS.
Structure 2b explains the low reactivity observed for the molecule
thought to have structure 2a’. Nevertheless, because some
biradical reactivity was observed for this species (reaction
efficiency 1%; Table 1), the ion population must still contain a
small amount of biradical 2a’. Similarly, 1a’ is proposed to
undergo cyclization in the third CAD step, followed by elimination
of the remaining iodine atom (bond dissociation energy[40] > 60
kcal mol-1) and generation of the 2-dehydro-4,5-oxygen-
peribridged quinolinium cation (1b; Scheme 3). The reactivity
observed for this radical cation (Table 1) is almost identical to that
of monoradical 3a, in agreement with the proposed structure (1b).
Not surprisingly, biradical 2a’ reacts with DMDS similarly as
7a, showing ●SCH3 abstraction as well as abstraction of both
●SCH3 and a H atom (total HSCH3 abstraction). However, the
reaction efficiency measured for 2a’ is very low (1%) compared to
all other species studied here (Table 1), despite its small
(calculated) S-T splitting (-0.9 kcal mol-1; Table 1) and higher
(calculated) vertical electron affinity than those for 3a – 6a (Table
1). In general, the smaller the S-T splitting and the greater the EA
for a singlet biradical, the faster are its radical reactions.[10,14–17,20]
The surprisingly low reactivity of 2a’ suggests that the ion may
not have the expected structure. Hence, the reactivity of its
immediate precursor, the 5-nitro-4-dehydroquinolinium cation (a;
Figure 4), was examined (see Supporting Information for details).
Unexpectedly, this ion was also found to react with DMDS at a
very low efficiency (0.2%) by ●CH3 (26%) and ●SCH3 radical
abstraction (14%) and by formation of a stable adduct (60%). The
2-iodo-5-nitro-4-dehydroquinolinium cation (analogous precursor
for 1a’) also reacts at a low efficiency (0.5%) via the same three
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10.1002/ejoc.201601572
European Journal of Organic Chemistry
FULL PAPER
Experimental Section
General Experimental Procedures
Most chemicals and solvents were obtained from commercial
sources and used without purification. 5-Nitroquinoline (5; Figure 1) was
purchased from Sigma-Aldrich. All other radical precursors (1 – 4, 6 and
7; Figure 1) were synthesized and characterized by both high resolution
mass spectrometry (elemental compositions were determined) and NMR.
The structures of previously unreported compounds (for 7, its 2-bromo-
precursor) was confirmed by x-ray crystallography (see Supporting
Information). All gas-phase reactivity studies were performed using a
Thermo Scientific linear quadrupole ion trap (LQIT) mass spectrometer
equipped with a manifold for introduction of reagents, as described
previously.[41,42] All radical precursors were introduced into the LQIT and
protonated by using the atmospheric pressure chemical ionization (APCI)
method. Radical sites were generated by using collision-activated
dissociation (CAD) of protonated radical precursors with helium collision
gas to cleave C-I, C-NO2 and/or O-NO bonds. The CAD process was
performed in multiple steps. Each CAD experiment resulted in the
cleavage of a single atom (e.g., I) or a single group of atoms (e.g., NO or
NO2). Hence, generation of bi- and triradicals required two or three distinct
CAD events, respectively. The charged radicals were isolated by ejecting
all unwanted ions from the ion trap and their reactivity toward dimethyl
disulfide (DMDS) was examined. The reaction efficiencies and reaction
products were determined. The efficiency of each reaction (i.e., the fraction
of collisions that leads to reaction) is given by kexp/kcoll, where kexp
represents the experimental reaction rate constant and kcoll represents the
theoretical collision rate constant calculated using a parameterized
trajectory theory.[42] The relative abundances of the primary products are
reported as branching ratios, which are given as the ratio of the abundance
of a primary product ion to the sum of the abundances of all products.
Scheme 2. Cyclization of 4-oxyl-5-dehydroquinolinium cation 2a’ to form 4,5-
oxygen-peribridged quinolinium cation 2b. Free energies (G298) calculated at
the M06-2X/6-311+G(d,p)//M06-2X/6-311+G(d,p) level of theory
Scheme 3. Proposed product ions formed upon sequential elimination of an I
atom, an NO● radical, and an I atom from 2,4-diiodo-5-nitroquinolinium cation
Very fast ●SCH3 abstraction indicates the presence of one highly
reactive carbon-centered σ-radical site in 1b.
Peribridged naphthalene and quinoline systems are of
interest[28–35] because they possess a strained four-membered
ring, which opens upon nucleophilic attack to yield new
naphthalene derivatives in solution.[30] Numerous research efforts
(both theoretical and experimental) have been performed on new
classes of peribridged naphthalenes containing carbon,[29]
boron,[31] phosphorus,[33] sulfur[28,30,31] and nitrogen[31,35] as the
peribridged atoms. However, only limited experimental evidence
exists for oxygen-peribridged napthalenes.[29,36] One such species
was reported[37] in 1933 but the results were later proven to be
erroneous.[38] A second study was published in 1971;[39] however,
the evidence did not permit distinguishing between a pair of
Computational Details
Density functional theory (DFT) calculations were carried out with
the Gaussian 09 electronic structure program suite.[43] Geometries for
radicals 1b, 2a–7a, 2a’ and 2b (Figure 2) were computed by using DFT
with the correlation–consistent polarized valence–triple– (cc–pVTZ[44]
)
interconverting
biradical
and
oxygen-peribridged
basis set. These DFT calculations use the gradient–corrected exchange
functional of Becke,[45] which is combined with the gradient-corrected
correlation functional of Lee, Yang and Parr[46] (B3LYP). All DFT
geometries were verified to be local minima by computation of analytic
vibrational frequencies, and these (unscaled) frequencies were used to
compute zero–point vibrational energies (ZPVE) and 298 K thermal (H298
– E0) and free energy (G298 – E0) contributions for all species. DFT
calculations for triplet and doublet states employed an unrestricted
formalism. For singlet biradicals 2a, 6a and 7a, the DFT “wave function”
naphthalene.[36,39] Therefore, this study presents the first
unambiguous experimental identification of an oxygen-
peribridged naphthalene derivative.
Conclusions
We report here a novel way of generating an oxygen
peribridged quinolinium cation and its monoradical counterpart in
the gas phase in a linear quadrupole ion trap mass spectrometer.
Based on the computational and experimental results described
here, the 2,4-diiodo-5-nitroquinolinium cation precursor (1) likely
generates the 2-iodo-4-oxyl-5-dehydroquinolinium cation upon
the first two CAD steps as a result of iodine atom and NO● losses.
The third CAD event involves ring closure (ΔG‡ = ca. 14 kcal mol-
1 based on the calculations for 2a’; Scheme 2) followed by loss of
the remaining iodine atom to yield a distonic oxygen-peribridged
quinolinium cation with a radical site at C-2 (1b). Observation of
only typical radical reactions (characteristic of 3a) for 1b suggests
that the oxygen-peribridged ring in 1b is substantially less reactive
than the radical site. Indeed, the even-electron oxygen-
peribridged quinolinium cation 2b shows very low reactivity
toward dimethyl disulfide.
was allowed to break spin symmetry by using an unrestricted formalism.[47–
51]
Because B3LYP is known to underestimate barrier heights,
molecular geometries for 5-nitro-4-dehydroquinolinium cation (a; Figure 4),
4-oxy-5-dehydroquinolinium cation (e; Figure 4), tricyclic structure (c;
Figure 4), nitrous oxide, 4,5-oxygen peribridged quinolinium cation 2b
(Scheme 2) as well as the transition states between a and c (b; Figure 4),
and c and e (d; Figure 4), were also optimized at the M06-2X level of
theory[52] by using the 6–311+G(d,p) basis set.[53] For the bond dissociation
energy calculations for 4-iodo-5-nitroquinolinium cation, geometries were
optimized at the M06-2X level of theory by using the 6-311G(d,p) basis
set.[53,54] All M06-2X geometries were verified to be local minima (or
transition states) by computation of analytic vibrational frequencies, and
these (unscaled) frequencies were used to compute zero–point vibrational
energies (ZPVE) and 298 K thermal contributions ((H298 – E0) and (G298
–
E0)) for all species. M06-2X calculations for the monoradicals, biradicals
and the transition states employed an unrestricted formalism.
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10.1002/ejoc.201601572
European Journal of Organic Chemistry
FULL PAPER
In order to compute vertical electron affinities for 2a’ and 6a – 7a,
second order multiconfigurational perturbation theory (CASPT2) single–
point calculations (CASPT2/cc-pVTZ) using the B3LYP/cc–pVTZ
optimized geometries were also carried out for the states that are produced
when a single electron is added to the nonbonding -orbital of the
monoradical (3a – 5a), or one of the nonbonding -orbitals of the biradical
(2a’, 6a and 7a; singlet ground state).[55] Thus, these calculations were
carried out for (zwitterionic) singlet states (monoradicals) or doublet states
(biradical).[56] The vertical electron affinities of the mono- and biradicals
[22] M. Holčapek, R. Jirásko, M. Lísa, J. Chromatogr. A 2010, 1217, 3908–
3921.
[23] K. Levsen, H.-M. Schiebel, J. K. Terlouw, K. J. Jobst, M. Elend, A. Preiß,
H. Thiele, A. Ingendoh, J. Mass Spectrom. 2007, 42, 1024–1044.
[24] A.-C. Schmidt, R. Herzschuh, F.-M. Matysik, W. Engewald, Rapid
Commun. Mass Spectrom. 2006, 20, 2293–2302.
[25] P. Hemberger, G. da Silva, A. J. Trevitt, T. Gerber, A. Bodi, Phys. Chem.
Chem. Phys. 2015, 17, 30076–30083.
[26] N. M. Donahue, J. S. Clarke, J. G. Anderson, J. Phys. Chem. A 1998,
102, 3923–3933.
were computed either as [E0(monoradical; doublet state)]
–
[E0(monoradical + electron; singlet state)] or [E0(biradical; singlet state)] –
[E0(biradical + electron; doublet state)]. Note that because these are
vertical electron affinities, zero–point vibrational energies (ZPVEs) and
298 K thermal contributions to the enthalpy are not included. The CASPT2
calculations included all  and nonbonding  electrons and molecular
orbitals in the reference wave function. CASPT2 calculations were carried
out using the MOLCAS electronic structure program suite.[57]
[27] R. G. Keesee, A. W. Castleman, J. Phys. Chem. Ref. Data 1986, 15,
1011.
[28] J. Nakayama, T. Fukushima, E. Seki, M. Hoshino, J. Am. Chem. Soc.
1979, 101, 7684–7687.
[29] R. J. Bailey, H. Shechter, J. Am. Chem. Soc. 1974, 96, 8115–8116.
[30] J. Meinwald, S. Knapp, S. K. Obendorf, R. E. Hughes, J. Am. Chem. Soc.
1976, 98, 6643–6649.
[31] I. Novak, L. J. Harrison, W. Li, B. Kovač, J. Phys. Chem. A 2007, 111,
2619–2624.
[32] A. Zweig, A. K. Hoffmann, J. Org. Chem. 1965, 30, 3997–4001.
[33] T. Mizuta, Y. Iwakuni, T. Nakazono, K. Kubo, K. Miyoshi, J. Organomet.
Chem. 2007, 692, 184–193.
Acknowledgements
This material is based upon work supported by the National
Science Foundation under Grant Number CHE-1464712. We
thank Dr. Phillip Fanwick and Dr. Matthias Zeller for assistance
with X-ray crystallography.
[34] H. Roohi, F. Deyhimi, A. Ebrahimi, J. Mol. Struct. Theochem 2001, 540,
287–294.
[35] I. Yavari, D. Nori-Shargh, J. Phys. Org. Chem. 1995, 8, 696–698.
[36] V. V. Mezheritskii, in Adv. Heterocycl. Chem., Elsevier, 2007, pp. 1–25.
[37] E. Ayers, British Patent 394511 (June 29, 1933).
[38] A. Gordon, J. Org. Chem. 1970, 35, 4261–4262.
[39] D. C. De Jongh, G. N. Evenson, Tetrahedron Lett. 1971, 12, 4093–4096.
[40] L. M. Kirkpatrick, N. R. Vinueza, B. J. Jankiewicz, V. A. Gallardo, E. F.
Archibold, J. J. Nash, H. I. Kenttämaa, Chem. – Eur. J. 2013, 19, 9022–
9033.
Keywords: radical reactivity · reaction mechanisms · oxygen-
peribridged quinoline · gas-phase ion-molecule reactions
[1]
[2]
[3]
[4]
[5]
[6]
W. Sander, Acc. Chem. Res. 1999, 32, 669–676.
H. H. Wenk, M. Winkler, W. Sander, Angew. Chem. 2003, 42, 502–528.
K. C. Nicolaou, W. M. Dai, Angew. Chem. 1991, 30, 1387–1416.
H. Pellissier, M. Santelli, Tetrahedron 2003, 59, 701–730.
K. Matyjaszewski, J. Xia, Chem. Rev. 2001, 101, 2921–2990.
W. Liu, S. D. Christenson, S. Standage, B. Shen, Science 2002, 297,
1170–1173.
[41] S. Gronert, J. Am. Soc. Mass Spectrom. 1998, 9, 845–848.
[42] T. Su, W. J. Chesnavich, J. Chem. Phys. 1982, 76, 5183–5185.
[43] Gaussian 09, Revision E.01, M. J. Frisch, G. W. Trucks, H. B. Schlegel,
G. E. Scuseria, M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone,
B. Mennucci, G. A. Petersson, H. Nakatsuji, M. Caricato, X. Li, H. P.
Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng, J. L. Sonnenberg, M.
Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T.
Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, J. A. Montgomery,
Jr., J. E. Peralta, F. Ogliaro, M. Bearpark, J. J. Heyd, E. Brothers, K. N.
Kudin, V. N. Staroverov, R. Kobayashi, J. Normand, K. Raghavachari, A.
Rendell, J. C. Burant, S. S. Iyengar, J. Tomasi, M. Cossi, N. Rega, J. M.
Millam, M. Klene, J. E. Knox, J. B. Cross, V. Bakken, C. Adamo, J.
Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin, R.
Cammi, C. Pomelli, J. W. Ochterski, R. L. Martin, K. Morokuma, V. G.
Zakrzewski, G. A. Voth, P. Salvador, J. J. Dannenberg, S. Dapprich, A.
D. Daniels, Ö Farkas, J. B. Foresman, J. V. Ortiz, J. Cioslowski, D. J.
Fox, Gaussian, Inc., Wallingford CT, 2009.
[7]
[8]
[9]
S. G. Weirschke, J. J. Nash, R. R. Squires, J. Am. Chem. Soc. 1993, 115,
11958–11967.
R. Hoffmann, A. Imamura, W. J. Hehre, J. Am. Chem. Soc. 1968, 90,
1499–1509.
P. G. Wenthold, R. R. Squires, W. C. Lineberger, J. Am. Chem. Soc.
1998, 120, 5279–5290.
[10] P. E. Williams, B. J. Jankiewicz, L. Yang, H. I. Kenttämaa, Chem. Rev.
2013, 113, 6949–6985.
[11] G. Wittig, Angew. Chem. Int. Ed. Engl. 1965, 4, 731–737.
[12] R. G. Miller, M. Stiles, J. Am. Chem. Soc. 1963, 85, 1798–1800.
[13] A. T. Bowne, T. A. Christopher, R. H. Levin, Tetrahedron Lett. 1976, 17,
4111–4114.
[44] T. H. Dunning, J. Chem. Phys. 1989, 90, 1007–1023.
[45] A. D. Becke, Phys. Rev. A 1988, 38, 3098–3100.
[46] C. Lee, W. Yang, R. G. Parr, Phys. Rev. B 1988, 37, 785–789.
[47] C. J. Cramer, S. Debbert, Chem. Phys. Lett. 1998, 287, 320–326.
[48] J. Gräfenstein, E. Kraka, D. Cremer, Chem. Phys. Lett. 1998, 288, 593–
602.
[14] B. J. Jankiewicz, A. Adeuya, M. J. Yurkovich, N. R. Vinueza, S. J.
Gardner, M. Zhou, J. J. Nash, H. I. Kenttämaa, Angew. Chem. Int. Ed.
2007, 46, 9198–9201.
[15] B. J. Jankiewicz, J. N. Reece, N. R. Vinueza, J. J. Nash, H. I. Kenttämaa,
Angew. Chem. Int. Ed. 2008, 47, 9860–9865.
[49] J. Gräfenstein, A. M. Hjerpe, E. Kraka, D. Cremer, J. Phys. Chem. A
2000, 104, 1748–1761.
[16] V. A. Gallardo, B. J. Jankiewicz, N. R. Vinueza, J. J. Nash, H. I.
Kenttämaa, J. Am. Chem. Soc. 2012, 134, 1926–1929.
[17] J. Gao, B. J. Jankiewicz, J. Reece, H. Sheng, C. J. Cramer, J. J. Nash,
H. I. Kenttämaa, Chem. Sci. 2014, 5, 2205–2215.
[50] T. D. Crawford, E. Kraka, J. F. Stanton, D. Cremer, J. Chem. Phys. 2001,
114, 10638–10650.
[51] V. Polo, E. Kraka, D. Cremer, Theor. Chem. Acc. 2002, 107, 291–303.
[52] Y. Zhao, D. G. Truhlar, Theor. Chem. Acc. 2008, 120, 215–241.
[53] R. Krishnan, J. S. Binkley, R. Seeger, J. A. Pople, J. Chem. Phys. 1980,
72, 650–654.
[18] P. F. Britt, A. I. Buchanan, E. A. Malcolm, J. Org. Chem. 1995, 60, 6523–
6536.
[19] S. E. Tichy, E. D. Nelson, F. S. Amegayibor, H. I. Kenttämaa, J. Am.
Chem. Soc. 2004, 126, 12957–12967.
[54] M. N. Glukhovtsev, A. Pross, M. P. McGrath, L. Radom, J. Chem. Phys.
1995, 103, 1878–1885.
[20] N. R. Vinueza, E. F. Archibold, B. J. Jankiewicz, V. A. Gallardo, S. C.
Habicht, M. S. Aqueel, J. J. Nash, H. I. Kenttämaa, Chem. – Eur. J. 2012,
18, 8692–8698.
[55] Note that, for these calculations, we are computing the vertical electron
affinity of the radical site, not the vertical electron affinity of the molecule.
[56] Because the mono- and biradicals studied here contain a formal positive
charge on the nitrogen atom, the states that are produced when an
[21] S. J. Blanksby, G. B. Ellison, Acc. Chem. Res. 2003, 36, 255–263.
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10.1002/ejoc.201601572
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electron is added to the nonbonding orbital are formally zwitterionic (i.e.,
they contain localized positive (π) and negative (σ) charges).
[57] Molcas 7.4: F. Aquilante, L. De Vico, N. Ferre, G. Ghigo, P.-A Malmqvist,
P. Neogrady, T. B. Pedersen, M. Pitonak, M. Reiher, B. O. Roos, L.
Serrano-Andres, M. Urban, V. Veryazov, R. Lindh, J. Comp. Chem. 2010,
31, 224-247.
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10.1002/ejoc.201601572
European Journal of Organic Chemistry
FULL PAPER
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COMMUNICATION
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Key Topic*
Radical reactivity
Author(s), Corresponding Author(s)*
Raghavendhar R. Kotha, John J. Nash,
and Hilkka I. Kenttämaa*
Collision-activated loss of two iodine atoms and a nitroso radical generated an
oxygen-peribridged quinolinium radical cation.
Page No. 1 – Page No. 5
Title
An Oxygen-Peribridged Quinolinium
Cation and Its Monoradical
Counterpart
*one or two words that highlight the emphasis of the paper or the field of the study
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